The crystal structure of Tl3Bi2I9: A complex defect and deformation variant of the perovskite structure type

Black crystals of trithallium(I) enneaiododibismuthate(III) Tl3Bi2I9 were obtained by slow cooling of a melt with composition T1 : Bi : I = 3 : 2 : 9 from 800 K to ambient temperature. The compound is diamagnetic and decomposes peritectically at 619(2) K. X-ray diffraction of single-crystals revealed that TI3Bi2I9 crystallizes in the monoclinic space group P2(1)/n with lattice parameters a = 1438.9(1), b = 771.54(7), e = 2091.0(1) pm and beta = 93.309(6)degrees, (Z = 4). The crystal structure can be described as a defect variant of the perovskite type (ABX(3)), where according to the formula TI(Bi-2/3 square(1/3))I-3 the Bi atoms occupy only two thirds of the octahedral sites B. The vacancies are located in every third plane resulting in double layers of corner sharing [Bil(6)] polyhedra. By distortion and rotation of the [BiI6] polyhedra the coordination of the TI atoms (cuboctahedral sites in the perovskite type, c.n. = 12) is reduced to bicapped trigonal prisms (c.n. = 8). The group-subgroup relation between the Tl3Bi2I9 structure and the perovskite type results in a subgroup index of 144.